A method for measuring a flow velocity of a fluid in a pipeline known in the art is described below. The method implies probing a controlled volume of a fluid with ultrasonic pulses emitted by a first source (emitter) and travelling transversely to the pipeline axis. The pulses that have passed through a controlled volume are registered by a first pulse receiver located oppositely to the emitter. A second pair of an emitter and a pulse receiver is located downstream at a known distance from the first pair. Using a cross correlation method applied to the signals of the two pulse receivers, the time interval in which the fluid flows from the first pair to the second pair can be determined. From this, the flow velocity is determined.
A disadvantage of this method is that measuring the flow velocity using the cross correlation method is not possible in an unsteady flow, as in this case the cross correlation method is often inaccurate. This yields an inaccurate determination of the flow velocity.